The present invention relates to an overload protective apparatus utilizing a bimetal for use in electric motors or the like.
Generally, a variety of products using electric motors, represented by refrigerators, air conditioners, dehumidifiers and so on, are provided with an overload protective apparatus for preventing the electric motor from being overheated and damaged. A number of propositions have conventionally been made for this type of overload protective apparatuses, examples of which may be those disclosed in JP-U-59-72641, JP-U-64-35642 and so on. Such an exemplary overload protective apparatus will be explained below with reference to FIGS. 1 and 2. FIG. 1 is a longitudinal sectional view of the example, and FIG. 2 is a plan view taken from the parting line II--II in FIG. 1. The illustrated overload protective apparatus comprises a case 1; an external bottom face 1a, an internal bottom face 1b; a lid 2; movable contacts 3, 4; a bimetal 5; an adjusting bolt 6; a head 6A of the adjusting bolt 6; fixed contacts 7, 8; fixed terminals 9, 10; a heater terminal 11; a heater line 12; and a spring 13.
In FIG. 1, the case 1 is made of a heat resistant material such as synthetic resin including phenol resin, unsaturated polyester and so on, and is formed in a cylindrical shape having a basal plane. The case 1 is covered with the lid 2 so that an internal space is formed thereby.
In the internal space, the adjusting bolt 6 made of brass is mounted in substantially the center of the bottom of the case 1 in a manner that it penetrates the bottom wall from the internal bottom face 1b to the external bottom face 1a. The adjusting bolt 6 has the head 6a at the extreme end thereof inside the case 1. The bimetal 5 in an arcuate dish shape is mounted to the adjusting bolt 6 which penetrates a supporting hole formed through the bimetal 5. The spring 13 is interposed between the bimetal 5 and the internal bottom face 1b of the case 1 such that the bimetal 5 is urged toward the head 6a of the adjusting bolt 6 by an urging force of the spring 13, whereby the bimetal 5 is spaced apart from the bottom of the case 1.
The bimetal is arcuate, and the pair of movable contacts 3, 4 are secured thereon by resistance welding symmetrically about the supporting hole.
The fixed contact 7 is secured at the tip of the fixed terminal 9 which penetrates the bottom wall of the case 1 from the internal bottom face 1b to the external bottom face 1a and is fixed on the internal bottom face 1b. This fixed contact 7 is positioned on the internal bottom face 1b opposite to one of the movable contacts indicated by 3 on the bimetal 5. Similarly, the fixed contact 8 is secured at the tip of the fixed contact 10 which is likewise fixed on the internal bottom face 1b and outwardly projects through the bottom wall. The fixed contact 8 is positioned on the internal bottom face 1b opposite to the other movable contact 4 on the bimetal 5.
Further, the heater terminal 11 is likewise secured on the internal bottom face 1b of the case 1, with a part thereof being projecting outwardly through the bottom wall of the case 1. Then, the heater line 12 is connected between the heater terminal 11 and the fixed terminal 9 by welding or the like. The fixed terminal 10 and the heater terminal 11 serve as external terminals of the overload protective apparatus. The heater line 12 is disposed close to the lower surface of the bimetal 5 such that it surrounds the adjusting bolt 6, whereby the whole bimetal 5 is heated by heat generated by the heater line 12.
The bimetal 5 is formed in a circular arc, the center of which is located in a central portion thereof. Specifically, when its temperature is low, it is curved with the central portion thereof projecting upwardly, as illustrated, so that the movable contacts 3, 4 are kept in contact with the fixed contacts 7, 8, respectively. Thus, a current path is formed from the fixed terminal 10 to the heater terminal 11 through the fixed contact 8, movable contact 4, bimetal 5, movable contact 3, fixed contact 7, fixed terminal 9 and heater line 12. As the bimetal 5 is heated and reaches a predetermined temperature, it suddenly changes its shape to an arcuate state with the central portion thereof projecting downwardly, i.e., reverse to the illustrated state. This sudden change in shape will hereinafter be called "the reverse motion" and the state of the bimetal 5 after the reverse motion will be called "the reverse state". Also, a temperature at which this reverse motion occurs will be called "the reverse motion temperature". When the bimetal 5 performs the reverse motion, the movable contacts 3, 4 are detached from the fixed contacts 7, 8, respectively, to break the current path.
When the temperature of the bimetal 5 begins falling due to the bimetal 5 being in the reverse state, and reaches a certain temperature, the bimetal 5 is restored to the initial state as illustrated. This motion will hereinafter be called "the restoring motion", and the illustrated state will be called "the original state". Also, a temperature at which the restoring motion occurs is called "the restoring motion temperature". When the bimetal 5 is restored from the reverse state to the original state, the movable contacts 3, 4 are again brought into contact with the fixed contacts 7, 8, respectively to restitute the current path.
FIG. 3 is a connection diagram showing an electric circuit when the foregoing overload protective apparatus is employed in an electric motor. The circuit includes an overload protective apparatus 14 as described above; an electric motor 15; a starter 16; a starting winding 17; and a main winding 18. Parts corresponding to those shown in FIGS. 1 and 2 are designated the same reference numerals.
As can be seen, FIG. 3 only shows a current path constituting portion in the overload protective apparatus 14 and the windings 17, 18 of the electric motor 15. The electric motor 15 has a series circuit formed by the starting winding 17 and the starter 16 connected in parallel with the main winding 18. The electric motor 15 is serially connected with the overload protective apparatus 14 by connecting one terminal of the electric motor 15 with the heater terminal 11 of the overload protective apparatus 14, whereby a current flows through the starting winding 17 and the main winding 18 of the electric motor 15 through the fixed terminal 10, bimetal 5, heater line 14 and heater terminal 11 of the overload protective apparatus 14.
In operation of the electric motor 15, if the electric motor 15 is mechanically locked by burning of a bearing portion in the electric motor 15 or a compressor, not shown, driven by the electric motor 15, invasion of contaminants or the like into rotating portions of the compressor, the rotor of the electric motor 15 is hindered from rotating, so that a large current corresponding to a starting current keeps flowing through the electric motor 15. This large current keeps flowing as long as the power supply is connected and the rotor remains locked. This large current is called "a constraint current" and amounts to four-five times a rated current of the electric motor 15. Normally, since the starting current flows only for a short period of 2-3 seconds (starting period) in a regular starting operation, the electric motor 15 is designed to be sufficiently resistant to a current of this magnitude flowing for such a short period of time. However, the electric motor 15 is not designed in consideration of the constraint current which keeps flowing through the electric motor 15 and current circuits associated therewith for a long period of time, so that such a state, not contemplated, is of course not preferable.
When a large constraint current flows through the electric motor 15, self-heating amounts of the bimetal 5 and the heater line 12 increase. Then, when the temperature reaches a reverse motion temperature of the bimetal 5, the bimetal 5 suddenly performs a reverse motion at that moment, whereby the movable contacts 3, 4 are detached from the fixed contacts 7, 8, respectively, resulting in breaking a current to the electric motor 15. If this breakage of the current takes place, the bimetal 5 and the heater line 12 begin to cool down. Afterward, when the temperature reaches the restoring motion temperature of the bimetal 5, the bimetal 5 suddenly performs a restoring motion to be restored to the original state, whereby the movable contacts 3, 4 are again brought into contact with the fixed contacts 7, 8, respectively, resulting in resuming applying a current to the electric motor 15.
At the time the electric motor 15 is again applied with a current, if the electric motor 15 has been released from a constraint state, the electric motor 15 performs a normal operation without the bimetal 5 again performing the reverse motion.
Next, another example of prior art overload protective apparatuses, as described in JP-U-60-183349 and so on, will be explained with reference to FIG. 4. Note that parts in FIG. 4 corresponding to those in FIG. 1 are designated the same reference numerals.
The illustrated prior art example basically differs from the example shown in FIG. 1 in that no heater line is provided in the former. For this reason, as shown in FIG. 4, a fixed terminal 9 having a fixed contact 7 at the tip thereof outwardly projects through a bottom wall of a case 1. Another fixed terminal 10 similarly projects outwardly, and these fixed terminals 9, 10 serve as external terminals of the overload protective apparatus. When movable contacts 3, 4 are in contact with fixed contacts 7, 8, a current path is formed from the fixed terminal 10 to the fixed terminal 9 through the movable contact 8, bimetal 5, movable contact 3 and fixed contact 7.
For employing the overload protective apparatus constructed as described above in an electric motor 15, one of the fixed terminals indicated by 9 of the overload protective apparatus 14 is connected to one terminal of the electric motor 15, as shown in FIG. 5.
If a large constraint current flows through the electric motor 15 due to a certain failure occurring therein, self-heating of the bimetal 5 increases. When the temperature reaches a reverse motion temperature of the bimetal 5, the bimetal 5 suddenly performs a reverse motion at that moment, whereby the movable contacts 3, 4 are detached from the fixed contacts 7, 8, thus breaking a current to the electric motor 15. When the breakage of the current takes place, the bimetal 5 begins cooling down. Then, when the temperature reaches a restoring motion temperature of the bimetal 5, the bimetal 5 suddenly performs a restoring motion to be restored to an original state, whereby the movable contacts 3, 4 are brought into contact with the fixed contacts 7, 8, respectively, resulting in resuming application of a current to the electric motor 15.
At the time the electric motor 15 is again applied with a current, if the electric motor 15 has been released from a constraint state, the electric motor 15 performs a normal operation without the bimetal 5 again presenting the reverse motion.
As described above, according to the respective prior art examples, if the electric motor 15 is released from a constraint state while the bimetal 5 remains in the reverse state, the electric motor 15 will resume a normal operation, thus preventing the electric motor 15 from being overheated and damaged.
However, if an abnormal state of the electric motor 15 has not been solved, and the electric motor 15 still remains in a constraint state when the bimetal 5 has performed the restoring motion to be restored to the original state, a large constraint current again flows into the overload protective apparatus 14, causing the bimetal 5 to again perform the reverse motion to get into the reverse state, thereby breaking a current to the electric motor 15.
As described above, as long as the abnormal state of the electric motor 15 is not solved, the bimetal 5 alternately repeats the reverse motion and restoring motion. When these operations are repeated a great number of times, the bimetal 5 will be gradually fatigued and finally broken. The aforementioned JP-U-60-183349 employs, as the bimetal 5, a member which is formed with a plurality of slits 5c extending radially from a supporting hole 5b into which an adjusting bolt 6 is inserted, as shown in FIG. 6. If the bimetal 5 as illustrated repeats a reverse motion and a restoring motion as described above, breaks E, F as illustrated will run from two end portions of the slits 5c.
It should be noted that the plane shape of the bimetal 5 shown in FIG. 6 is such that rectangular end faces 5a, 5a' project from two opposite locations on the outer periphery 5d of a region formed substantially in a circular or elliptic shape. The movable contacts 3, 4 are secured by resistance welding at positions symmetric about the supporting hole 5b near the end faces 5a, 5a'.
In an overload protective apparatus for turning a large current on and off, since large movable contacts 3, 4 are employed suitable for the magnitude of the current, few freedom is ensured during the reverse motion for portions of the bimetal 5 for coupling the movable contacts 3, 4 thereto, whereby stress in these portions will increase. For this reason, breaks may advance simultaneously from surroundings of the movable contacts 3, 4.
If the bimetal 5 is broken as described above, the characteristics of the bimetal 5 will change to cause decreases in pressures against the contacts and a contact separating force as well as changes in the reverse motion temperature and restoring motion temperature, with the results that, even if the bimetal 5 performs the reverse motion, time intervals between the reverse motions become shorter due to some causes such as a decreased displacement amount of portions of the bimetal 5 for coupling the movable contacts 3, 4 by the reverse motion of the bimetal 5, whereby a conductive ratio of the constraint current flowing through the bimetal 5 and the heater line 12 is increased to cause the temperature inside the case 1 to rise more and more.
A final failure caused by such repetitive motions is contact deposition between the movable contacts 3, 4 and the fixed contacts 7, 8. If the contact deposition occurs in this manner, a large constraint current will continuously flow through the windings of the electric motor 15 and the bimetal 5 of the overload protective apparatus 14, causing the windings of the electric motor 15 to be heated and damaged, or the temperature inside the case 1 to rise due to heated bimetal 5 and heater line 12. If a resultant temperature inside the case 1 exceeds a resistible temperature of the case 1 and the lid 2, this will result in also burning or damaging members near the bimetal 5 such as the case 1, the lid 2 and so on.
Incidentally, in the prior art example shown in FIG. 1, if the heater line 12 is cut by an abnormally rising temperature inside the case 1, the current path of the overload protective apparatus 14 is also broken, so that the above-stated burn and damage can be prevented, thus ensuring the security. However, the heater line 12 is not always cut every time the temperature inside the case 1 becomes abnormally high. Thus, the heater line 12, not cut in such a situation, will cause a problem on the security. With the overload protective apparatus 14 as shown in FIG. 4, which does not even have a heater line, this action cannot even be expected.
Generally, a fusing current of a bimetal in an overload protective apparatus for use in a refrigerator is 70 amperes or more for five seconds of conduction. On the other hand, that for use in an air conditioner and so on is 100 amperes or more. These values indicate that the bimetal will not be ruptured unless a current two times or more of a maximum constraint current of a electric motor for use in these machines flows through the bimetal.
A variety of methods for solving the problems stated above have been proposed. As an example of these methods, JP-U-59-72641 discloses a case made of a heat resistant material such as ceramics.
JP-U-63-174145 discloses a method, where a motion frequency indicating plate having a plurality of saw-tooth-like protrusions is provided such that a bimetal sequentially engages with a different saw-tooth protrusion every time the bimetal performs a restoring motion to gradually lower the motion frequency indicating plate, and when the bimetal has repeated the restoring motion the number of times equal to the number of saw-tooth-like protrusions, the motion frequency indicating plate abuts to the internal bottom face of a case so as to prevent the bimetal from further performing the restoring motion. According to this method, even if an abnormal state of an electric motor is not solved, the bimetal is inhibited from performing the restoring motion after repeating it predetermined times, and it is maintained in the reverse state to break a constraint current.
JP-A-63-224125 discloses a method, where a first bimetal and a second bimetal having a reverse motion temperature higher than that of the first bimetal are connected in series in a manner that if an abnormal current occurs, the first bimetal first performs a reverse motion, and if the abnormal state still remains unsolved after the first bimetal has repeated the reverse motion and restoring motion, and the first bimetal is finally broken to cause contact deposition, an abnormal temperature rise resulting from the contact deposition causes the second bimetal to perform a reverse motion to break the abnormal current.
JP-U-64-1450 discloses a technique of arranging a second bimetal on the lower surface of a first bimetal such that if the first bimetal is broken to cause contact deposition, the second bimetal performs a reverse motion to lift up the first bimetal.
The aforementioned JP-U-64-35642 also discloses that the head of the adjusting bolt, to which the bimetal is mounted, is made as a separate part from the adjusting bolt, and is formed with a recess which is filled with a thermally soluble metal when the head is fitted on the adjusting bolt, such that the head is bonded on the tip of the adjusting bolt with this thermally soluble metal. While the bimetal is normally urged to the head by the spring, if the bimetal is broken to cause contact deposition and consequently heated, the thermally soluble metal is fused to release the bonding between the head and the adjusting bolt, whereby the bimetal and the head are lifted by an urging force of the spring.
JP-A-3-77228 discloses a method, where elongated holes 502 are formed through a bimetal at locations between a supporting hole 501 of the bimetal and two movable contacts 3, 4 to provide higher resistance portions having an electric resistance higher than that of the bimetal, so that Jour heat generated by eddy currents is concentrated on these higher resistance portions, thus rupturing the bimetal in the higher resistance portions. JP-A-3-77228 also describes that, similar effects can be produced by reducing the thickness of partial regions of the bimetal or providing recesses in the periphery of the bimetal, as alternative methods of providing the higher resistance portions.
Next, description will be made as to another overload protective apparatus which detects an abnormal state in which no eddy current is generated.
A separate type air conditioner, for example, has an indoor machine and an outdoor machine coupled to each other through pipes. If the piping is incompletely made, Freon or a coolant may leak. In this case, a compressor of the air conditioner will be excessively heated, while a current through an electric motor employed therein does not substantially increase from a no-load current, so that conventional protectors responsive only to a current, as disclosed in JP-A-59-72461 and so on, cannot protect the electric motor for this case.
Conventionally, to attend to this problem, a hermetic type protector is disposed inside a compressor to directly detect a temperature of the compressor, as disclosed in JP-U-60-95183, JP-U-62-38090, JP-Y2-63-5422, JP-A-63-61783 and so on. However, although this type of protector can prevent burn and damage caused by overheating due to the above-stated leak of a coolant, the hermetic type protector itself is very expensive, and moreover its mounting is so complicated that the number of manufacturing processes is increased, thus causing an increase in the manufacturing cost.
Also, since the protector is built in a compressor, when a failure occurs in the protector, the whole compressor must be subjected to a service for replacing the protector, thus incurring a disadvantage of increasing a service expenditure.
As a means to solve these problems, JP-A-2-139820 discloses an overload protective apparatus for mounting to an outer shell of an electric motor for driving a compressor, i.e., a two-element type thermal protector. Such a two-element type thermal protector is provided with a series circuit of a heater R and a temperature switch TH on an open end side of a cylindrical case which accommodates a main protector P constituted of a dish-shaped main bimetal 19 and so on, as shown in FIG. 8. The heater R is disposed near the main protector P.
The two-element type thermal protector as described above is disposed between a power supply switch SW and an electric motor M. In a normal state, the main protector P is closed, while the temperature switch TH is opened. Thus, the electric motor M is supplied with a driving current from a power supply, not shown, through the power supply switch SW and the two-element type thermal protector. If the temperature of a compressor rises due to an accident such as a leak of a coolant, the temperature switch TH detects this temperature rise and closes, as shown in FIG. 9A, to make the heater R conductive so that heat is generated from the heater R. By the heat from the heater R, the main bimetal of the main protector P is heated and opens contacts, as shown in FIG. 9B, to break the current to the electric motor M.
While a variety of methods for attending to contact deposition of a bimetal have been proposed as described above, they further imply the following problems.
If a case is formed of ceramics as described in JP-U-59-72641, although it is ensured that the case is protected from being burnt or damaged, windings of an electric motor, serving as a load, cannot be saved from the burn and damage. A further problem is that the case becomes expensive.
In a conventional mechanism which is provided with an operation frequency indicating plate as described in JP-U-63-174145, the number of times of reverse and restoring motions of a bimetal is limited by this operation frequency indicating plate, so that the following problems are left unsolved for putting this type of mechanism into practice:
(1) An overload protective apparatus for use in a refrigerator, air conditioner, dehumidifier or the like inevitably operates in response to failures of an electric compressor, i.e., failures caused by other than a mechanical lock, whereby a bimetal is quite easily held in a reverse state by the operation frequency indicating plate, thus resulting in an increase in demanding maintenance services. PA1 (2) The operation frequency indicating plate advances the number of repeated reverse motions even for confirming operations in an adjustment procedure, thus decreasing the remaining number of times allowed to the reverse motion. PA1 (1) A range of the magnitude of an available current is limited depending on the specific resistance of these bimetals. PA1 (2) When the specific resistance of the bimetals is short and a sufficient heat amount cannot be generated thereby, a heater line must be additionally provided. However, since an insulation distance must be maintained between the bimetals and the heater line, a space occupied by the heater line is also expanded, resulting in a larger size of the overload protective apparatus. PA1 (3) Each of the first and second bimetals requires expensive contacts to be disposed thereon, which makes the overload protective apparatus extremely expensive. PA1 (1) When contact deposition occurs on a bimetal and the bimetal is heated to a high temperature, the thermally soluble metal begins melting and the head is finally separated from the adjusting bolt, so that the bimetal and the head of the adjusting bolt are lifted up by a spring. However, this lifting-up is advanced slowly due to the viscous property of the thermally soluble metal. Subsequently, when the lifted bimetal causes movable contacts to be detached from fixed contacts on the internal bottom face of a case, the current path is broken, so that the heat source is lost simultaneously with the breakage, whereby the thermally soluble metal directs to a solid phase. Nevertheless, if the urging force of the spring does not act to sufficiently overcome the viscous property of the thermally soluble metal, a sufficient contact separating amount (contact gap) cannot be ensured between the movable contacts and the fixed contacts when the bimetal is lifted up as described above. PA1 (2) The above stated solid phase phenomenon of the thermally soluble metal is nothing but a load resistance for the spring, and acts to decrease a contact separating force of the spring at the time contact deposition occurs. It is therefore expected that the solid phase phenomenon constitute an obstacle to providing an overload protective apparatus for turning on and off a load through which a large current will flow. PA1 (3) Since a creep exists in the bonding of the head with the adjusting bolt with a thermally soluble metal, a sufficient temperature difference should be ensured between the melting point of the thermally soluble metal and a reverse motion temperature of a bimetal. Generally, a required temperature difference for this case is approximately 40.degree.-50.degree. C. For this reason, an operation temperature at which contacts are detached is elevated, so that an available range of the apparatus may be limited in many applications. PA1 (4) A highly stable facility is required for applying a melt thermally soluble metal into a recess in the head of the adjusting bolt, so that the facility cost for manufacturing this type of overload protective apparatus becomes unacceptably high. PA1 (1) A larger adjustment margin must be provided for the bimetal. PA1 (2) A stress of the bimetal is increased to cause a break to initiate from the higher resistance portion, which results in accelerating fatigue of the bimetal and consequently shortening the life. PA1 (3) In the case of the bimetal formed with throughholes or recesses, the life of the bimetal is acceleratively shortened by a notch effect. PA1 (4) In the method of partially reducing the thickness of a bimetal, the life of the bimetal is increasingly shortened by a machining effect exerting on a wide region of the bimetal and by peeling of layers in a different metal bonded portion (a laminated portion). PA1 (1) As is well known, contact deposition is more likely to frequently occur due to a decrease in contact pressure. PA1 (2) As a result, the life of a bimetal, particularly for use in turning on and off a large capacitive load, is significantly shortened, whereby the function of the bimetal will be disabled at an earlier time. PA1 (1) Since elongated holes such as 502 shown in FIG. 7 exist between the movable contacts and the radial slits, a spring constant of the bimetal will be decreased, whereby the basic characteristics of the bimetal are largely changed. PA1 (2) Since the radial slits causes a current path resistance of the bimetal to become larger, the length of the elongated holes must be further extended in order to provide the higher resistance portions at locations close to the movable contacts rather than locations near the central supporting hole. Consequently, the spring constant of the bimetal becomes smaller, causing the contact pressure to be lower and the movable and fixed contacts to be more susceptible to contact deposition, whereby such a bimetal is not acceptable in a practical use. PA1 1. As is well known, the moment the bimetal 19 is opened to break a current to the electric motor M for driving the compressor, a surge voltage de is generated as represented by the following equation: EQU de=di/dt PA1 2. When the bimetal is restored to the original state to close the circuit, its main contacts present not a few mechanical vibrations (i.e., chattering or bouncing), so that the electric motor M for driving the compressor is repetitively turned on and off, though for a short period of time. Also in this event, a surge voltage is generated as described above, and if the temperature switch TH is closed, a majority of the surge voltage is applied to the heater R. PA1 (1) A bimetal is provided with a plurality of throughholes around movable contacts such that the cross-sectional area of a current path of the bimetal becomes smaller near the movable contacts. The throughholes may be radially formed from the center of each of the movable contacts or concentrically arranged around the same. PA1 (2) Movable contacts are welded on a bimetal with small welding areas such that the welded portions are ruptured when an abnormal current flows through the bimetal. PA1 (3) The width of movable contact bonding portions on a bimetal is made substantially identical to the diameter of movable contacts such that a current density becomes particularly larger near the movable contact bonding portions. PA1 (4) Movable contacts bonded on a bimetal are made of a material having different electric conductivity and thermal conductivity from those of the bimetal. PA1 (5) The volumes of a pair of movable contacts are made different such that a current density in one of them is larger than the other. PA1 (6) The areas of end faces of a bimetal on which movable contacts are to be bonded are made different from each other such that different current densities are given to the bimetal from the respective movable contacts. PA1 (7) A plurality of slits are radially formed from a supporting hole provided at the center of a bimetal, where at least a pair of slits located at opposite positions to each other with respect to a straight line connecting movable contacts are elongated to vicinities of the periphery of the bimetal in comparison with the other slits. With such a structure, a current density is made larger in portions of current paths between the elongated slits and the outer periphery of the bimetal, the cross-sectional area of which is smaller, whereby if an abnormal current flows, the bimetal is ruptured at these portions. PA1 (8) A bimetal is generally formed in a two-layer structure including a metal layer having a higher expansion coefficient and a metal layer having a lower expansion coefficient, with the exception that portions of the bimetal around movable contacts are formed in a single-layer structure consisting only of a metal having a higher or a lower expansion coefficient. Since the movable contacts are also made in a single-layer structure, the current density is higher in these portions, and accordingly greater amounts of heat are generated in these portions. Thus, if an abnormal current flows, these single-layer structure portions are ruptured. PA1 (9) A polymetallic elememt, e.g., a trimetal is generally formed in a three-layer structure including a metal layer having a higher expansion coefficient, a metal layer having a lower expansion coefficient, and an intermediate metal layer sandwiched by the first two metal layers, with the exception that portions of the polymetallic element around movable contacts are formed in a single-layer structure consisting only of a metal having a higher or a lower expansion coefficient. PA1 (10) A polymetallic element having a combination of the respective characteristics described above. For example, a bimetal may be provided with a plurality of throughholes around movable contacts such that a current path resistance becomes larger near the movable contacts, as well as with a plurality of slits extending radially from a supporting hole formed at the center of the bimetal, where at least a pair of slits located at opposite positions to each other with respect to a straight line connecting movable contacts are elongated to vicinities of the periphery of the bimetal in comparison with the other slits. PA1 1. A highly safe overload protective apparatus can be realized without changing the conventional basic structure. PA1 2. The polymetallic element of the present invention may be employed in all overload protective apparatuses without increasing the number of elements and irrespective of the presence or absence of a heater, so that the overload protective apparatuses can be manufactured at a lower cost. PA1 3. Since there is no negatively acting factor for breaking a circuit, the polymetallic element of the present invention may be employed in all overload protective apparatuses for use in a range from a small current to a large current. PA1 1. A small and cheap heat generating element can be used for a combination with the main polymetallic element. From the fact that the volume of the heat generating element can also be adjusted arbitrarily, a good thermal response is provided. PA1 2. A small and cheap temperature switch may also be arbitrarily selected, and its responsibility is made excellent. PA1 3. Accordingly, the two-element type thermal protector comprising these elements can also be made compact, cheap and excellent in operability. PA1 4. As a result, the two-element type thermal protector provides a protection characteristic which not only covers a wide range similar to the conventional like protectors but also is largely improved, so that thermal damage, which may possibly be given to an electric motor for driving a compressor or a load, can be reduced, and moreover a high reliability is held for a long term. PA1 5. The present invention can be easily applied to a variety of apparatuses without changing the conventional basic structure, thus providing large practical effects.
When serially connected first and second bimetals are used as described in JP-A-63-224125, since these bimetals must be simultaneously conducted, the following problems are left unsolved for putting this technique into practice:
When a head of an adjusting bolt is bonded to the adjusting bolt with a thermally soluble metal as described in JP-U-64-35642, the following problems are left unsolved for putting this technique into practice:
When throughholes are formed through a bimetal so as to provide the bimetal with higher resistance portions having a higher electric resistance than the remaining region of the bimetal, as described in JP-A-3-77228, these holes should be located near movable contacts on the bimetal as is apparent from FIG. 7.
When recesses are formed in the periphery of a bimetal, although their positions are not disclosed in JP-A-3-77228, it can be said that these recesses are necessarily formed near movable contacts for a reason similar to the above stated case of forming throughholes.
Further, for reducing the thickness of a bimetal, although no disclosure is given with respect to where and how to thin the bimetal, it is easily estimated with respect to the positions that regions near movable contacts will be thinned similarly to the case of forming the throughholes. With respect to the method, it can be thought to partially thin a bimetal by pressing or the like. However, in any cases, when a resilient force of a bimetal acts on movable contacts, a deformation amount of the bimetal inevitably concentrates on the higher resistance portions near the movable contacts, the cross-section of which is minimal. Therefore, if the same contact pressure is to be ensured as compared with the prior art, the following problems can be foreseen:
To avoid the above problems, if a bimetal is to be put into practice with a lower contact pressure compared with the prior art:
Further, if a bimetal intended to decentrate stresses of the bimetal as disclosed in JP-U-60-183349 (FIG. 6) and the bimetal proposed by JP-A-3-77228 (FIG. 7) are combined for a practical use, the following problems may arise:
Additionally, in this prior art, the bimetal supporting mechanism does not have a spring for urging the bimetal to the head of the adjusting bolt, so that the bimetal, after ruptured, may freely take a position, incurring a fear that a sufficient distance for separation cannot be ensured between the fixed contacts and the movable contacts on the bimetal.
With the two-element type thermal protector shown in FIGS. 8, 9A and 9B, when an accident such as a leak of a coolant causes the temperature in a compressor to rise, the temperature switch TH detects heat radiated from a dome of the compressor. When the detected temperature reaches a predetermined value, the temperature switch TH closes the current path to conduct and heat the heater R, whereby the main bimetal 19 of the main protector P is heated to open the current path. Therefore, when the state shown in FIG. 8 is changed through the state shown in FIG. 9A to the state shown in FIG. 9B, the following problems may be thought:
where de is a circuit voltage and dt is a breaking time.
The surge voltage generated in this event is applied to a closed circuit including the electric motor M for driving the compressor, temperature switch TH and heater R, where a majority of the voltage is applied to the heater TH having a large impedance.
It should be noted that the surge voltage de generated by the electric motor M for driving the compressor may reach in general several hundred to one thousand and several hundred volts.
From the foregoing, the heater R is required to have an overvoltage characteristic which can bear such a large surge voltage. This is because if the heater R could not bear this large voltage, the heater R would be cut in the worst case, whereby functions as the two-element type thermal protector cannot be achieved.
However, if selection is made to the heater R in view of the overvoltage characteristic bearable to the surge voltage, a selected heater will be larger and expensive.